This invention relates to an electrochemical process for the production of high-purity chromic acid (CrO.sub.3) comprising the following steps:
1. preparing and purifying an aqueous sodium chromate/sodium dichromate solution,
2. converting the sodium chromate/sodium dichromate solution into a sodium dichromate/chromic acid solution with a molar ratio of sodium ions to chromic acid of 0.45:0.55 to 0.30:0.7 by multistage membrane electrolysis,
3. crystallizing solid chromic acid from this sodium dichromate/chromic acid solution by evaporation to a water content of approximately 9 to 20% by weight and preferably 12 to 15% by weight H.sub.2 O at temperatures in the range from 55.degree. C. to 110.degree. C.,
4. separating the chromic acid crystallized out from the mother liquor by centrifugation and washing out of the adhering mother liquor with a substantially saturated chromic acid solution having a temperature of at least 55.degree. C. and removing the washing solution by centrifugation,
5. recirculating the mother liquor separated off in the centrifuge into a middle stage of the multistage membrane electrolysis mentioned in 2. and, at the same time, removing a small amount of the mother liquor to remove impurities from the electrolysis/crystallization circuit.
Chromic acid (CrO.sub.3) is industrially produced by three different processes:
In the so-called melt process, sodium dichromate crystals are reacted with concentrated sulfuric acid in a molar ratio of approximately 1:2 at temperatures of around 200.degree. C. In the so-called wet process, sulfuric acid and sodium dichromate are combined with one another in concentrated aqueous solution. In both processes, sodium bisulfate contaminated with chromium is unavoidably formed either as a melt or as an aqueous solution.
This disadvantage and the accompanying losses of chromium is avoided by the third process, namely the membrane electrolysis of sodium dichromate in aqueous solution. The electrochemical process, which is described for example in Canadian patent specification A-739,447, is based on the principle common to membrane electrolyses using a cation-selective membrane, namely the migration of the cations in an anode compartment through the cationselective membrane forming the dividing wall between the anode and cathode compartments into the cathode compartment under the effect of the electrical field.
Embodiments of the electrochemical process for the production of chromic acid are described in Canadian patent specification A-739,447. From a sodium dichromate solution introduced into an anode compartment, the sodium ions in the electrical field migrate through the membrane into the cathode compartment filled with water or aqueous solution and, with the hydroxide ions formed at the cathode with evolution of hydrogen, form an aqueous solution containing sodium ions while, in the anode compartment, the dichromate ions remaining behind are electrically neutralized by the hydrogen cations formed at the anode with simultaneous evolution of oxygen.
Broadly speaking, therefore, this process comes down to the substitution of the sodium ions in the sodium dichromate by hydrogen ions, i.e. to the formation of chromic acid. During the conversion of the sodium dichromate solution into a sodium dichromate solution containing an increasing quantity of chromic acid, the migration of the sodium ions through the membrane is increasingly accompanied by the migration of the hydrogen ions formed in the anode compartment, so that the utilization of the electric current for the desired removal of sodium from the anode compartment, also known as the current efficiency, steadily decreases. This means that the sodium dichromate cannot be completely converted into chromic acid in the anode compartment, and the conversion is only operated to an average degree on economic grounds. The chromic acid then has to be separated off from these solutions by fractional crystallization, leaving a mother liquor containing the sodium dichromate which has not been electrochemically converted and residues of non-crystallized chromic acid. This solution is conveniently introduced into the electrolysis process for further conversion into chromic acid. The following problems ensue from these process principles: on the one hand, the mother liquor adhering to the chromic acid crystals and consisting of almost concentrated sodium dichromate solution has to be carefully washed to obtain a pure product; on the other hand, all impurities introduced with the sodium dichromate solution collect in the system and are ultimately discharged with and in the chromic acid crystals because only the electrolysis gases, hydrogen and oxygen, leave the process and the membrane separating off the anode compartment is largely impermeable to anions and also to polyvalent cations. Accordingly, it is not possible by this process to obtain high-purity chromic acid. In addition, cationic impurities in the sodium dichromate solution introduced, particularly polyvalent cations, lead to premature exhaustion and destruction of the membrane separating the anode and cathode compartments, probably because precipitations of insoluble hydroxides and salts of these cations occur as a result of the major pH changes taking place within the membrane in very thin layers.
DE-A 3 020 261 describes a process for electrochemical production of chromic acid from dichromate, of which the object is to enable the production of chronic acid to be carried out with high current efficiency and to eliminate the impurities introduced with the dichromate.
The process according to DE-A 3 020 261 is essentially characterized by the use of a three-compartment cell, the dichromate solution entering the middle compartment and leaving it again in dichromate-depleted form and, as it flows through, releasing sodium ions to the cathode compartment separated off by a cation-selective membrane and dichromate ions to the anode compartment separated off by a diaphragm or an anion-selective membrane. Although it is possible in this way to produce a chromic acid solution substantially free from impurities, a high voltage is required for the electrolysis process on account of the large electrode intervals enforced by the middle compartment. Accordingly, this process is unsatisfactory on account of the complicated and vulnerable three-compartment structure.
DE-A 3 020 260 describes the purification of sodium chromate solution for the electrochemical production of chromic acid. In this purification process, the sodium chromate solution is subjected to electrolysis in the anode compartment of a two-compartment cell with a cation-selective partition and the cationic impurities are precipitated in the membrane with simultaneous formation of sodium dichromate in the anode compartment and of an alkaline solution containing sodium ions in the cathode compartment, as known per se from U.S. Pat. No. 3,305,463. The sodium chromate/sodium dichromate solution thus purified is electrochemically converted into chromic acid in the manner described above. Two major disadvantages, namely the frequent replacement or purification of the very expensive membrane charged with the contaminated cations and the necessary conversion of the sodium chromate used into sodium dichromate solely with electric current rather than the considerably less expensive inorganic acids, sulfuric acid or carbon dioxide, make the proposed process economically unattractive.
Accordingly, the object of the present invention is to provide a process which, while retaining the advantages of the electrochemical production of chromic acid, enables a high-purity, crystalline chromic acid to be produced under economic conditions.